1. Field of the Invention
The present invention relates to a differential signal transmission cable that includes a pair of signal conductors and transmits differential signals having a phase difference of 180 degrees.
2. Description of the Related Art
Devices (e.g., servers, routers, and storage products) dealing with high-speed digital signals of several gigabits per second (Gbit/s) or more have adopted a differential interface standard, such as the low-voltage differential signaling (LVDS). Between devices or between circuit boards within a device, differential signals are transmitted through a differential signal transmission cable. Differential signals are characterized by having a high resistance to external noise while making it possible to provide a low-voltage system power supply.
A differential signal transmission cable includes a pair of signal conductors, which are configured to transmit a plus-side signal and a minus-side signal having a phase difference of 180 degrees. A potential difference between these two signals is represented by a signal level. For example, if the potential difference is plus, a signal level “High” is detected on the receiving side, and if the potential difference is minus, a signal level “Low” is detected on the receiving side.
Examples of the differential signal transmission cable that transmits such differential signals are disclosed in Japanese Unexamined Patent Application Publication Nos. 2011-086458, 2011-096574, and 2012-169251. The differential signal transmission cables disclosed in these documents include a pair of signal conductors arranged in parallel and spaced apart by a predetermined distance. Each of the signal conductors is covered by an insulator, and the entire periphery of the insulator is covered by a sheet-like shield conductor.
In the differential signal transmission cables disclosed in the documents described above, the entire periphery of the insulator is covered by a shield conductor. As a result, for example, a manufacturing error in the differential signal transmission cable may cause variation in distance from each signal conductor to the shield conductor.
FIG. 7 is a transverse cross-sectional view of a differential signal transmission cable of the related art having a manufacturing error. Specifically, as indicated by distances “e” and “f”, a differential signal transmission cable “a” has a difference in thickness dimension of an insulator “d” (i.e., a difference in distance to a shield conductor “g”) in the direction in which a signal conductor “b” on the positive (P: +) side and a signal conductor “c” on the negative (N: −) side are arranged. That is, in the case of FIG. 7, the distance “e” between the signal conductor “b” on the P side and the shield conductor “g” is shorter than the distance “f” between the signal conductor “c” on the N side and the shield conductor “g” (e<f).
The difference in thickness dimension of the insulator “d” (i.e., the difference between the distances “e” and “f”) leads to a difference in dielectric constant ∈ and further leads to a difference in effective dielectric constant ∈ef between the signal conductor “b” on the P side and the signal conductor “c” on the N side. The difference in effective dielectric constant ∈ef between the signal conductors “b” and “c” causes a difference in propagation time between transmission signals propagating through the signal conductors “b” and “c”. The effect of a so-called “skew” on transmission signals becomes more significant as the speed of the signals increases. Therefore, there has been a need to change the structure of differential signal transmission cables to support high-speed transmission signals.